The invention relates to a steam injection heater of the type disclosed in U.S. Pat. No. 2,455,498 and U.S. Pat. No. 3,984,504. These heaters are desirably provided with a steam regulating piston as disclosed in these patents. When using the heater to heat slurries with solids with large particle sizes, the solids can enter the space between the piston skirt and cooperating chamber wall to interfere with smooth operation of the piston. The heaters illustrated in the foregoing patents also are not specifically designed for sanitary heating applications. The components cannot be readily disassembled for cleaning purposes.